1. Field of the Invention
This invention pertains to transparent bodies formed from polymers. More particularly, compositions and methods for forming are provided for transparent polymer bodies that are resistant to fouling by algae and other organisms.
2. Description of Related Art
Solid, impact-resistant, optically transparent materials are used as covers for remote optical sensors, cameras, and other instruments employed for monitoring conditions in marine and freshwater environments. The covers are used to isolate the instruments from the environment while allowing sensing of light. The sensors may be in place for extended periods of time and should be maintenance-free. To achieve this goal, the transparent materials used as covers need to resist or eliminate biofouling (unwanted attachment of biological organisms to the surface of the transparent material). Biofouling may be caused by algae, bacteria, mold, and other biological agents. Other potential applications of solid transparent materials that are resistant to biofouling include aquarium windows, greenhouse glass, and building glazing material. All such materials used to transmit light are referred to herein as “coverglasses.”
An antifouling (AF) agent is defined here as a chemical compound that prevents the growth of organisms on the surface where the agent is present. Ideally, the AF agent is environmentally benign. In less-than-ideal cases, a suitable AF must degrade rapidly in a neighboring environment and/or have controlled release partitioning in the environment resulting in limited bio-availability to other organisms. Use of an AF agent must result in minimal accumulation of toxic compounds (i.e., toxic antifoulants must chemically degrade rapidly in the environment) and the AF agent must have limited toxicity to organisms in the local environment surrounding the protected object at concentrations that are effective at providing antifouling action. The primary marine AFs used extensively over the past several decades have been in the family of organotins. Chief among these are the tributyltin (TBT) compounds. While these compounds were extremely effective as antifoulants, they have proven to be too toxic and too long-lived. As a result, TBTs were outlawed by the International Maritime Organization effective Jan. 1, 2003.
Less toxic marine AF alternatives have since been approved and put into use. These include: various copper compounds (copper (I) oxide, copper thiocyanate, metallic copper, copper bronze, copper napthenate, and other copper compounds), SeaNine 211 (4,5-dichloro-2-moctyl-4-isothiaz-olin-3-one; Rohm and Haas), Irgarol® 1051 algaecide (N-(tert-butyl)-N′-cyclopropyl-6-(methylthio)-1,3,5-triazine-2,4-diamine; Ciba Specialty Chemical), and Diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea; DuPont Chemical). While all of these materials are attractive as being more environmentally-friendly than TBT, they still are not completely environmentally benign.
A new class of antifoulants—natural product antifoulants (NPA)—is gaining more attention and use. The most studied NPAs are capsaicin (the principal “hot” ingredient in peppers) and zosteric acid (ZA, ρ-coumaric acid sulfate, a naturally occurring phenolic acid found in eel grass). Both of these compounds have shown good antifouling properties, particularly against marine animals. In previous antifouling studies, NPAs have been placed into various compounds including paints, silicone, and polystyrene. It has been shown in head-to-head comparisons that capsaicin displays more effective antifouling properties than zosteric acid. However, due to its molecular structure, capsaicin is able to leach out of an encapsulating matrix more quickly into the environment. As a result, the lifetime of protective layers which incorporate capsaicin into the matrix are shorter than that observed with zosteric acid.
Antibiofouling agents have been added to polymers, paints, films and laminates. For example, U.S. Pat. No. 6,607,826 discloses a plastic anti-fouling film for boat hull protection. Transparent materials have also had metal- and non-metal-based antifoulants placed on them as protective films; however, these materials are protective coatings and are not distributed throughout the whole window or coverglass material.
U.S. Pat. No. 6,608,129 discloses a polymer matrix having “natural free volume” in which is placed inorganic or organic materials that may be antifouling. A composite or surface layer is formed. The polymer is evacuated before the polymer is exposed to the active antifoulant material, which may polymerize or react with the polymer. The disclosed polymers were not designed to be transparent to light nor were they designed to be impact resistant.
U.S. Pat. No. 6,635,692 discloses extruded polymers incorporating antifouling agents, including organic biocides. Antifoulant release from different types of polymers and from polymers with different antifoulant loadings is reported in the patent. The polymers were extruded as narrow sheets using a counter-rotation twin screw extruder fitted with a slit die. The disclosed polymers were not designed to be transparent to light nor were they designed to be impact resistant.
Immersion of transparent polymers in surfactant solutions was also found to decrease growth of bacteria. (Kerr, A., et al, ′The Biofouling Resistant Properties of Six Transparent Polymers with and without Pre-treatment by Two Antimicrobial Solutions,” Materials and Design, Vol. 22, pp 383-392, June 2001) This treatment is a coating on the polymers and is not a biocide nor is it distributed throughout the polymer system.
What is needed is a transparent polymer having good impact resistance that can be made resistive to fouling organisms without significantly reducing the optical transmission of the polymer. The material providing the fouling resistance should not leach into an environment to an extent that causes environmental concerns.